The present invention relates to a composite cord for the reinforcement of elastomers such as rubber, for example used for tires, conveyor belts and hoses.
Steel cords and more particular compact cords are widely known. They are used for the reinforcement of rubber articles. In a compact steel cord the composing steel filaments have the same twisting direction and the same twisting step. The filaments of a compact steel cord have line contact with adjacent steel filaments. Both steel cords and compact cords have the drawback that the surface of the core filament can be damaged by fretting. This damage can be considerably high. The fretting is not limited to the core filaments, also the filaments arranged around the core filament suffer from fretting.
In the conventional cord a single filament is wrapped around said cord. This type of cord features the disadvantage that the wrapping filament causes fretting on the filaments of the outer layer. EP 0 627 520 provides a wrapless compact steel cord, whereby the outer filaments fulfil the function of the wrapping filament. However, these outer filaments exercise a great pressure on the core filament. This results in fretting on the core filament and thus in a considerable damage of this core filament.
Another known disadvantage of steel cords with a core and filament layers, and more particularly of compact cords, is that they suffer from core migration. Core migration is the slipping out of the filaments of the cord due to repeated bends.
It is an object of the present invention to provide a composite cord which avoids the drawbacks of the prior art.
It is also an object to reduce the fretting of the core filament and to reduce the fretting of the filaments arranged around the core filament. It is a further object to provide a composite cord with an improved stability of the cord structure. With an improved stability of the cord structure is meant a better and more stable distribution of the filaments over the cord. It is also an object to avoid core migration. It is still another object of the invention to provide a composite cord with an increased life time.
According to one aspect of the invention, there is provided a composite cord for reinforcement of elastomers comprising a core of a high polymer material. A first layer of steel filaments is twisted around said core and a second layer of steel filaments is twisted around said first layer. Preferably, all filaments of the first and the second layer have the same twisting step. More preferably, all filaments of the first and second layer have not only the same twisting step but also the same twisting direction.
Since the core filament consists of a polymer material, there is no fretting on the core filament. Core migration which is a considerable drawback of the prior art steel cords is by that also avoided.
In a preferred embodiment, the polymer material is present in a sufficient volume to create gaps between the filaments of the first layer. In such an embodiment there is no direct contact between the steel filaments of the first layer. This results in a considerable reduction of the degree of fretting between the filaments of the first layer.
Preferably, the gaps between adjacent filaments of the first layer have an average size of 0.002 mm. More preferably the average size of the gaps is at least 0.004 mm. By average size of the gaps is meant the average size of all gaps between adjacent filaments of the first layer over the entire length of the cord. Occasionally, some cross-sections may have gaps that are less than the average size.
The size of the gaps can be expressed in terms of the theoretical diameter of the central core d, the diameter of the filament of the first layer D and the number of filaments in the first layer n, according to the following equation:             a      ⁡              (                  d          +          D                )              ⁢    sin    ⁢                  180        ⁢        xc2x0            n        -  D
By theoretical diameter of the central core of the cord is meant the diameter of the best fitting circle in the hole created by the steel filaments of the cord.
In another embodiment, the volume of the polymer core is chosen in order to create gaps not only between adjacent filaments of the first layer but also between adjacent filaments of the second layer.
The presence of gaps between the steel filaments is important since it allows the penetration of the elastomer compound to the steel filaments up to the core of the cord. In this way the filaments can be completely embedded in the polymer material, i.e. the high polymer material of the core and the elastomer.
Different types of organic filaments can be used as core material. To avoid the melting of the polymer material during a preheating or during vulcanisation, the melting point of the used polymer must be sufficiently high. Preferably, the melting temperature of the polymer material is higher than 135xc2x0 C., for example higher than 140xc2x0 C. Preferably, the melting point is lower than the vulcanisation temperature. Such polymers only melt partially during the preheating; a completely flowing out is nevertheless avoided.
The polymer core comprises at least one filament of a high polymer. The filaments may be fused or twisted. Suitable filaments are polyamid filaments, polyester filaments, polyethylene filaments, polypropylene filaments, aramid filaments such as Twaron(copyright) or Kevlar(copyright), filaments made of a copolyester thermoplastic elastomer, such as Arnitel(copyright), or any strong filaments. Suitable polyamid filaments are for example nylon 6 or nylon 6.6. Also polyethylene naphtalate (PEN) or polyethylene terephtalate (PET) filaments can be considered.
Polyester has the advantage that it is characterised by a low adsorption of humidity. It has been shown that a core comprising one or more high performance polyethylene fibers is very suitable. These fibers have a tensile strength of more than 2 GPa, for instance 3 GPa. These super strong polyethylene fibers preferably have a macromolecular orientation of more than 85%, more preferably the macromolecular orientation is greater than 90%. The level of crystallinity of the polyethylene is preferably more than 80%, for instance 85%. These fibers are known as Dyneema(copyright).
Alternatively, filaments made of two different materials can be used as polymer core. These filaments comprise a core material coated with a polymer material. The core material gives the cord the required strength. The polymer material surrounding the core material preferably flows out during the heat treatment.
The core material can for example be PEN or PET. The material surrounding the core material is for example polyethylene, polypropylene or a copolyester thermoplastic elastomer, known as Arnitel(copyright). Also a modified PET characterised by a lower melting point than the conventional PET can be used as covering material of the core. These materials may be applied to the core material by extrusion.
For all the above mentioned polymer core material, the diameter of the polymer core is chosen so that the above mentioned minimum size of the gaps between adjacent filaments of the first layer is obtained.
The polymer filaments may be dipped in order to reach a good adhesion between the elastomer compound and the polymer.
Depending on the type of polymer that is used and depending on the diameter of the polymer core, different embodiments can be realised.
In a first embodiment a polymer with a melting temperature higher than the vulcanisation temperature is used as core. A polymer with such a high melting point is for example polyester. Because of the high melting temperature of the polymer, the polymer material does not flow out during the heating. Preferably, the diameter of the polymer core is chosen so that the filaments arranged around the core in a first layer do not touch each other.
In a second embodiment a polymer with a melting point higher than 140xc2x0 C. but lower than the vulcanisation temperature is used. During vulcanisation the polymer material flows out. The amount of polymer is chosen so that the material fills the central hole created by the filaments of the cord completely and the gaps between adjacent filaments of the first layer at least partially. The filaments of the first layer are thus at least partially embedded in a viscous or viscoelastic matrix. In this way the polymer material forms a kind of a cushion. An average gap size between adjacent filaments of the first layer of at least 0.002 mm is obtained, this means that the filaments have no direct contact with each other. Occasionally, the gap size can be less than the desired average gap size and in some exceptional cases there can be a cross-section where some filaments have contact with each other. But even in the latter situation, the average gap size is at least 0.002 mm. Because the partial embedment of the filaments in the polymer matrix and because there is no direct contact between adjacent filaments, there is a considerable improvement of the fretting characteristics of the filaments of the first layer. A composite cord according to the above described embodiment further features the advantage that the stability of the cord type is improved.
In a further embodiment, after vulcanisation the polymer material does not only fill the central hole of the cord and the gaps between adjacent filaments of the first layer completely, but the amount of polymer is chosen so that after flowing out of the polymer material, the polymer material has contact with the second layer of filaments. At least half of the number of the filaments of the second layer, i.e. the filaments which are situated between two filaments of the first layer, has contact with the polymer material. This further reduces the fretting of the filaments, not only of the filaments of the first layer, but also of the filaments of the second layer.
Possible constructions of cord types are C+6+12, C+8+12, C+8+13, C+9+15. In this notation C is the high polymer core. For compact cords, the number of filaments in the first layer is preferably 5, 6 or 7. The number of filaments in the second layer is preferably the double of the number of filaments in the first layer. Preferable constructions for compact cords are constructions of the type C+15, C+18 and C+21.
As mentioned above, preferably, all of the filaments are embedded in polymer material, either by flowing out of the high polymer material and/or by the penetration of the elastomer towards the center of the cord. The penetration of the elastomer can further be increased by replacing some of the filaments of the second layer by filaments having a somewhat lower diameter.
The composite cords are preferably wrapless cords. The filaments of the second layer exercise a force which is directed radially inward. In this way the filaments of the outer layer fulfil the function of the wrapping filament of a conventional steel cord. This radially inward directed force of the outer layer filaments can be realised in different ways. A first way to realise this is to provide the outer layer filaments with residual torsions which tend to close the steel cord. A second way to exert a force which is directed radially inward is to preform the outer layer filaments in such a way that the preforming ratio is smaller than or equal to one hundred percent. The preforming ratio of a particular filament is defined as the ratio of the diameter of the helicoid of the disentangled filament to the diameter of the helicoid of the filament in the cord. A third way to exert a force which is directed radially inward is to put the outer layer filaments in the steel cord under a tensile force. The outer layer filaments are put under a tensile force in the steel cord, if after careful disentangling of the outer filaments, the twist pitch of the disentangled outer filaments is smaller than the twist pitch of the steel cord. Also combinations of the three above described ways are possible in order to realise a radially inward directed force of the outer filaments. The way of manufacturing such a wrapless steel cord is disclosed in EP 0 627 520. Such kind of steel cord, however, suffers from fretting of the core filaments since there is a great pressure on this core filaments by the outer filaments.